1. Field of the Invention
The present invention provides a method to enhance step coverage of a metal film deposited into high aspect ratio features formed on a substrate.
2. Background of the Related Art
Physical vapor deposition (PVD) or sputtering is a known technique used in the manufacture of integrated circuits. Sputtering is a method by which material on a target are displaced to a desired surface of a substrate where they form a thin film. In a typical PVD process the target and the substrate to be coated are placed in a vacuum chamber which is evacuated to and maintained at a pressure of less than about 10 milliTorr. An inert gas, such as argon, is supplied to the vacuum chamber and a pumping system maintains the desired gas pressure in the chamber. A glow discharge plasma is created in the chamber by supplying a negative DC or RF potential to a cathode (typically the target) and grounding the chamber walls and an anode (typically the substrate). The glow discharge plasma is created in the space between the cathode and the anode, and is generally separated from the electrodes by a dark space or plasma sheath. In a standard PVD chamber, a dense plasma exists near the target. This plasma is maintained by secondary electrons emitted from the target during the sputtering process. Using a magnetron assembly the secondary electrons are trapped by magnetic fields to efficiently create a plasma adjacent the target. In this arrangement, an electric field is produced that is substantially perpendicular to the exposed surface of the target. Thus, positive ions from the plasma are accelerated across the dark space onto the exposed surface of the target resulting in sputtering of the target.
The goal in most deposition processes is to deposit a film of uniform thickness across the surface of a substrate, while also providing good fill of lines, interconnects, contacts, vias and other features formed on the substrate. In some applications, a conformal liner, barrier or seed layer may be deposited. For example, in a copper fill process a barrier layer is deposited on a feature formed in a substrate to prevent diffusion of copper into the base material of the substrate. Subsequently, a conformal seed layer is deposited over the barrier layer and copper is deposited to fill the feature. As device geometries shrink, it has become increasingly difficult to deposit materials conformally into small features to form barrier and seed layers in these features.
With recent decreases in the size of semiconductor devices and corresponding decreases in device features to less than a quarter micron ( less than 0.25 xcexcm) in aperture width, conventional sputtering (i.e., PVD) has been sheared through the use of a high density plasma (HDP) PVD process, known, for example, as ionized metal plasma (IMP) PVD. IMP-PVD uses a coil disposed between a sputtering target and a substrate to ionize atoms sputtered from the target. As the ionized metal atoms approach the plasma boundary near the substrate, the electric field caused by an applied bias on the substrate directs the ionized metal atoms in a direction generally perpendicular to the substrate surface. These ions are accelerated perpendicularly towards the surface of the substrate within the plasma sheath, improving the selective or preferential filling of high aspect ratio features, e.g., sub-quarter micron. Biasing of the substrate relative to plasma potential is widely used in HDP-PVD to control the energy of ions reaching the substrate and improve directivity. Because the ionized metal atoms are traveling normal to the surface of the substrate, they can deposit into the bottom of high aspect ratio features without hitting the sidewalls of the features and forming overhangs at the top comers of the features.
One of the problems with HDP-PVD is due to the relatively large difference in molar mass between the target material and the plasma gas. For example, the molar mass ratio of Copper to Argon is about 1.59. Because of this difference, target atoms cannot be readily ionized in the HDP-PVD chamber. In an attempt to increase ionization of the sputtered metal particles, it has been suggested to increase the chamber pressure, thereby increasing the plasma density. The higher density, in turn, reduces the mean free path between particles resulting in more collisions and increased ionization. However, the deposition results are compromised once the pressure reaches an upper limit.
Another problem with HDP-PVD is the inability to achieve conformal coverage in the increasingly smaller device features. Conformal coverage of the bottoms and sidewalls of the features is needed to optimize subsequent processes such as electroplating. Electroplating requires conformal barrier layers and conformal seed layers within the device features in order to ensure uniform filling of the feature. While conventional HDP-PVD processes achieve good bottom coverage due to the directionality of the ions provided by the bias on the substrate, the sidewall coverage is not as good. This result is caused in part by the induced high directionality of ions toward the bottoms of the features with little directionality toward the sidewalls.
Therefore, there is a need for a metal deposition process which provides conformal coverage in high aspect ratio features.
The present invention generally provides a method for depositing a generally conformal film on a substrate to form barrier layers and/or seed layers. The method includes deposition of a material at a first pressure followed by deposition of the material at a second pressure. In one embodiment, the first pressure is higher than the second pressure. The high pressure step results in relatively more deposition of a material on a feature bottom and the low pressure step results in relatively more deposition of material on the sidewalls of the feature.